The present invention is related generally to systems and methods for determining the range and bearing of the source of radiofrequency ("RF") signal and, in particular, to systems and methods for determining the range and bearing of such signals in the face of multipath and similar noise sources.
Systems and method for determining the distance and bearing of an RF signal are well known. In some systems, a outbound signal having a known power is sent from a base station to a remote station at an unknown location. The remote station may "respond" to the signal from the base station by returning a signal upon receipt of the outbound signal. Such a system is presently produced by Cubic Defense Systems, Inc. as the AN/ARS-6(V) PLS system or by Rockwell International Inc as the Target Locating System (TLS). The distance between the base station and the remote station may be determined by any of the known methods. For example, the distance can be computed by timing the total transit time between the transmission of the outbound signal and the receipt of the response signal. By subtracting the estimated time of the delay in the remote station from the total transit time, the time to traverse twice the distance between the base and the remote stations can obtained and the distance readily computed. By way of another example of prior art systems, the distance between the base and the remote stations can be estimated by knowing the power of the signal transmitted from the remote and measuring the power of the signal received at the base station. Using the inverse square law of signal strength over increasing distances, an estimate of the distance can be obtained from the difference between the transmitted power and the received power.
Likewise, it is known in the prior art to determine the direction of the response signal by one of many techniques. For example, in one of the most simple methods, a loop antenna may be rotated and the strength of the response signal measured. The transmitting station is estimated to be along the line corresponding to the axis of the loop when the loop is position to maximize response signal power. In another example in the prior art, a base station may use plural antennas having a known geometric relationship to one another. The angle of arrival of the response signal may be determined by evaluating the phase of the response signal simultaneously at each of the antennas. The simultaneous phase relationships at the antennas, the geometric relationship of the antennas and the frequency of the response signal can be used to estimate the angle of arrival of the response signal with respect to the antennas.
All of the above-noted systems and methods for determining range and bearing in the prior art experience some difficulty in multipath and other noisy environments typical of where many such tracking and ranging systems are used. For example, with reference to FIG. 1, an RF signal source 10 may be located at location remote and unknown to a base station 12. Plural blocking and/or reflecting elements 14, such as buildings, towers, mountains may exist in the proximity of and in the direct path between the RF signal source 10 and the base station 12. The blocking and/or reflecting elements cause RF signals impinging upon such elements to be blocked, absorbed, reflected, and often a combination of all three. Generally, such elements cause RF signals to be diminished in strength and to change direction. When a source of RF signals such as the remote source 10 radiates RF signals, such signals are blocked and/or reflected by the elements 14 such that instead of a single signal arriving at the base station 12, multiple versions of the same or slightly altered signal arrive at the base station 12. The different versions of the signals arrive at different times because they have travelled different paths of different distances than either the direct version or other indirect versions. The signals may also be altered from one another because each of the signals has experienced a different environment and may have been subject to different noise and interference sources along the different paths.
With continued reference to FIG. 1, in a multipath environment, the signal which arrives directly from the RF signal source 10 at the base station 12 may not be the strongest signal. For example, in the system of FIG. 1 three different paths 20, 22, and 24 between the RF signal source 10 and the base station 12 are shown. (It being understood that generally communications are conducted across an arc and not just at selected lines from the RF signal source.) The first signal path 20 proceeds directly from the RF signal source 10 to the base station 12. Because the first signal path intersects two of the elements, and each element tends to diminish the strength of the signal, the signal arriving at the base station 12 is lower in amplitude or power than a signal arriving without being partially absorbed. Note that when the signal on the first signal path impinged on the elements, it is likely that some portion of the signal was reflected and some portion was refracted and never reached the base station 12 but such reflection and refraction are not shown with respect to the first signal path 20 for simplicity of illustration.
The second signal path 22 in the illustration of FIG. 1 is reflected off two of the elements 14 before reaching the base station 12 (refraction and absorption not being shown). If the reflecting surfaces of these two elements are relatively efficient, a relatively strong signal will reach the base station 12 along the second signal path 22. Because the signal traveling the second signal path 22 travelled a longer distance than the signal travelling the first signal path 20, the signal on the second path will arrive at the base station 22 after the signal on the first signal path 20. Similarly, the third signal path 24 is reflected off an element 14 to reach the base station 12.
Note that in the system of FIG. 1, the various signals arrive at the base station from entirely different angles. In some systems in the prior art, the locating system will operate on the signal having the strongest signal power. As can be seen from the illustration in FIG. 1, such a procedure will lead to an erroneous result as the signal amoving with the strongest power arrives along the second signal path 22, from almost the very opposite of the actual angle to the RF signal source. Note also that if ranging is done on the basis of the strongest signal, the ranging determination will be in error because the strongest (second signal path 22) travels more distance than the distance between the RF signal source 10 and the base station 12.
The influences of multipath signals on distance and angle location has been recognized in the prior art. Some prior art systems ignore the influence of multipath by utilizing a composite signal based on the strengths of the various multipath signals identified by the base station 12. The systems of FIG. I illustrate how the composite signal may be erroneous as signal which is the composite of the arriving signals may yield a signal which is misaligned such as the composite signal 26. As can be seen from the illustration, the direct signal path (first signal path 20) will yield the best "direction" information but the "composite" signal 26 received by the base station is a combination of signals from different angles of arrival.
It is accordingly an object of the present invention to provide a novel system and method of tracking a remote RF transmitter which obviates these and other known problems in the prior art.
It is a further object of the present invention to provide a novel system and method of tracking a remote RF transmitter which has a reduced susceptibility to the effects of multipath.
It is another object of the present invention to provide a novel system and method of tracking a remote RF transmitter by determining the range and direction of an arriving signal with respect to the portion of the signal arriving directly from the RF transmitter.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.